The evolution of smaller and smaller mobile telephones and other portable radio receivers and transceivers wirelessly communicating personal digital assistants has also promoted the need for antennas that do not protrude outside the smooth contours of the device. However, having the antenna inside the housing of the mobile station easily causes interference problems between the antenna and other electronics of the mobile station.
FIG. 1 illustrates a cross section of a typical mobile station according to the prior art. The mobile station 100 has a helix antenna 101 which is connected to the printed wired board 103 together with other electronics 102. The battery 105 of the mobile station forms a part of the housing 104. On the front side of the mobile station there is a microphone 106, keyboard 107, display unit 108 and earpiece 109.
Patent document [1] U.S. Pat. No. 5,787,340 discloses an arrangement where a shelding layer is located beside the user and an antenna. Such a shielding layer is illustrated in FIG. 1 as 120. A similar solution is also disclosed in article [2] “Reduction of SAR in Human Head by Suppression of Surface Currents due to a Portable Telephone” by J. Wang and O. Gujiwara, IEICE Trans. Commun., vol. E80-B, pp. 1810–1815. This article discloses an arrangement with low reluctance ferrite sheet disposed on the housing of a portable telephone. These prior art methods can be used also for reducing interference of RF fields to the handset. However, while the part of the RF radiation is absorbed with the additional shield, a portion of the RF intensity is lost, which means that the efficiency of the antenna may not be optimal. This problem is partially solved in a solution disclosed in patent document U.S. Pat. No. 5,507,012 where reflecting material is used in addition to an absorbing RF shield. This solution, however, is relatively complicated and therefore it is not well suited for mass production. A further disadvantage with these prior art solutions is their limitation to devices where the antenna is located outside the housing of the mobile station. Except the antenna, also the shield is visible and makes a negative effect on the appearance and usability of the mobile station. If the antenna would be placed inside the housing there could be interference problems between the antenna and other electronics of the device.
Patent document [3] U.S. Pat. No. 5,982,335 discloses a a microstrip antenna with a ground plane, microstrip radiator and a substrate disposed between the radiator and the ground plane. Theory of microstrip antennas is also described in [4] “Small Antennas” ISBN 086380 048 3, pages 138–165.
The arrangement described in U.S. Pat. No. 5,982,335 includes low magnetic reluctance material on the surface of the ground plane. The purpose of the low magnetic reluctance material is thus to influence the radiation pattern of the antenna. It would be possible to use this kind of microstrip antenna inside a mobile station, but there are certain drawbacks related with this prior art solution as well.
Firstly, the dielectric substrate of a microstrip antenna takes a lot of space within the housing of the mobile station, and therefore the size of the housing would become too large, or the space available for other electronics would become too small. Another drawback is the fact that while the antenna would be located just next to the electronics circuits, the radiation of the antenna could cause RF interference to the electronics. The low magnetic reluctance material on the surface of the ground plane affects the radiation pattern of the antenna by, for example, attenuating the radiation intensity in the direction opposite to the radiator. However, a microstrip antenna tends to cause high radiation intensity in small areas in the nearest surrounding area of the antenna.
A further drawback is related to the fact that it is difficult to connect a microstrip antenna to the RF electronics of a mobile station. The antenna element must first be equipped with connecting wires, next the printed wired board must and the antenna element must be installed within the housing, and finally the antenna wires must be soldered to the printed wired board. It is quite difficult to arrange these production phases within an automated production line, and thus manual assembly work may be required.